1. Field of the Invention
The present invention relates to a method for making a field emission device used for various displays, light sources, high speed switching devices, microsensors and so on from which electrons are emitted by a field effect among electron sources. More particularly, it relates to a method for making a field emission cathode having a microtip.
2. Description of the Prior Art
Flat panel displays for wall television sets may be either a liquid crystal display (LCD), a plasma display panel (PDP), or a field emission device (FED). The field emission device may have very high luminous efficiency and luminescence by highly integrating the tips of the field emission material to 10.sup.4 -10.sup.5 Tips/m.sup.2, and thereby reducing voltage consumption.
FIG. 3 is a view illustrating a typical prior art structure of the field emission device. A reference numeral 31 indicates a substrate doped with impurities of high density and having high conductivity. A cathode made of molybdenum Mo serving as an electron emission device 36 is formed in a cavity 35 between insulating layers 34 on the substrate 31. In addition, a gate electrode 38, surrounding the cathode 36, and made of a molybdenum thin film is deposited on the insulating layers 34.
For example, by biasing the gate electrodes 38 within the range of tens to hundreds of volts to the substrate 31, an electronic field of about 10.sup.6 V/cm-10.sup.7 V/cm is generated between a tip of the cone-shaped cathode 36 and the gate electrodes 38, and thus an emission current of about several hundreds of mA can be obtained from the tip of the cathode 36.
FIG. 4 illustrates a perspective view of a prior art display using a field emission device as the electron source (refer to Japan Patent Unexamined Publication Sho 61-221783).
Referring to FIG. 4, a plurality of cathode electrodes 42 is formed on a lower glass 40 in accordance with the directions of rows 41, and a cone-shaped field emission device 46 and an insulting layer 44 are formed on the cathode electrode 42. Also, a plurality of gate electrodes 48 is formed on the insulting layer 44 in accordance with the directions of columns 45. Cavities or holes are formed at the opposite side of the cone-shaped field emission device 46 of the gate electrode 48.
Meantime, a transparent conductive layer 52 and a fluorescent layer 54 are respectively deposited to the lower glass 40 and to the upper glass 50 to be fixed in a beta configuration. The lower substrate 40 and the upper substrate 50 together with a spacer (not shown) form the outside of a vacuum-tube.
Positive electric potential is applied to the transparent conductive layer 52. Responsive to a display signal, a predetermined electric potential difference is given between the cathode electrode 42 in the rows 41 and the gate electrode 48 in the columns 45. An appropriate electric field is formed between the gate electrode 48 and the cone-shaped field emission cathode 46, such that electrons are emitted from a cone-shaped tip. When electrons are emitted from the cavity of the gate electrode 48 to the opposite fluorescent layer 54, the fluorescent layer 54 is excited and radiates. An image in accordance with the display signal is displayed by the above-mentioned operation.
By making the diameter of the tip of cathode 46 tens of namometers the prior art field emission device does not have any problems in forming the high field required for emitting the electrons from the tip of the cathode. However, there are disadvantages in the display operation. When the electron emission from the plurality of tips of the cathodes is induced, the tips of the cathodes break or wear out due to current concentration on a predetermined cathode. Further, there is the problem that brightness around a predetermined portion in the phosphor layer is abnormally greater than around the remaining portion. This creates an instability in emission brightness since electron emission power from the tip of the cathode is not stable. In addition, if the tip of the cathode falls off, the electron emission yield is reduced since during the etching process, etching material permeates into the contact portion between the cathode and the cathode electrode forming the cathode tip array.